Heterodontosaurus tucki (Crompton & Charig, 1962) ~1 m in length. Hettangian, Early Jurassic ~198 mya is the only ornithischian with fangs. Paleontologists have long considered it one of the most primitive ornithischians. Here Heterodontosaurus was derived from a sister to the new dinosaur, Daemonosaurus and Pisanosaurus. Heterodontosaurus phylogenetically preceded the bone-headed pachycephalosaurs and the horned ceratopsians and psittacosaurs likeYinlong.
Distinct from Daemonosaurus, the skull of Heterodontosaurus had a more down-curved premaxilla with shorter fangs. The skull was taller posteriorly. The postnarial process of the premaxilla was further enlarged until it reached the palpebral (eyebrow bone). The maxilla was shallower and concave ventrally. It did not descend as far as the premaxilla. The quadratojugal was enlarged and expanded to nearly cover the quadrate. The old ventral connection of quadratojugal to jugal was replaced by a new, more robust connection midway up. The jugal also developed a lateral bump below the orbit. The mandible was deeper. The mandibular fenestra was smaller. All of the maxillary and most of the dentary teeth were not sharp. However one fang remained at the anterior dentary and it tucked into a recess in the rostrum above. The tooth sets slid past each other like scissor/grinders.
Distinct from Lesothosaurus, the skull was relatively larger. The neural spines were longer and slightly taller, especially closer to the pelvis.
The forelimb was little changed, but the forearm and hand were larger. The chevrons were slightly longer.
The forearm and hand were relatively larger. Digit 4 was a vestige. Digit V was nearly absent.
The pelvis and hind limb were nearly identical to that of Lesothosaurus.
A sister taxon, Tianyulong (Zheng et al. 2009) preserved stiff bristles, perhaps similar to proto-feathers, over the pelvic area.